The present invention relates to preferential oxidation reactors and more particularly to a multi-stage preferential oxidation reactor.
Fuel cells have been proposed for many applications from electrical power plants to replacing internal combustion engines. Hydrogen is often used as the fuel and is supplied to an anode of the fuel cell. Oxygen (from air) is the oxidant and is supplied to a cathode of the fuel cell. A typical fuel cell is described in U.S. Pat. No. 5,316,871 to Swathirajan, et al.
The hydrogen used in the fuel cell can be derived from the reformation of methanol or other organics (e.g. hydrocarbons). Unfortunately, the reformate includes undesirably high concentrations of carbon monoxide (CO) which can quickly poison the fuel cell anode catalyst, and therefore, the CO must be removed. For example, in the methanol (CH3OH) reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen (H2) and carbon dioxide (CO2) according to the following reaction: CH3OH+H2Oxe2x86x92CO2+3H2.
This reaction is accomplished heterogeneously within a chemical reactor that provides the necessary thermal energy throughout a catalyst mass and yields a reformate gas comprising hydrogen, carbon dioxide, carbon monoxide and water. One such reformer is described in U.S. Pat. No. 4,650,727 to Vanderborgh. Carbon monoxide (i.e. about 1-3 mole %) is contained in the H2-rich reformate exiting the reformer and must be removed or reduced to a non-toxic concentration to avoid poisoning the anode.
It is known that the carbon monoxide level in the reformate can be reduced by utilizing the water-gas shift reaction. To achieve this, a water-gas shift reactor is provided, within which water (as steam) could be added to the methanol reformate stream exiting the reformer, to lower its temperature and to increase the steam-to-carbon ratio therein. A lower reformate temperature and higher steam-to-carbon ratio serve to decrease the carbon monoxide content of the reformate according the following ideal water-gas shift reaction: CO+H2Oxe2x86x92CO2+H2.
However, some carbon monoxide still remains after the water-gas shift reaction. Depending upon the reformate flow rate and the steam injection rate, the carbon monoxide content of the gas exiting the water-gas shift reactor can be as low as 0.5 mole %. Any residual methanol is converted to carbon monoxide and hydrogen in the shift reactor. Hence, the shift reactor reformate comprises hydrogen, carbon dioxide, water and some carbon monoxide.
The water-gas shift reaction does not sufficiently reduce to carbon monoxide content of the reformate (i.e., to below about 20 ppm) to avoid poisoning of the anode. Therefore, it is necessary to remove the remaining carbon monoxide from the hydrogen-rich reformate stream exiting the water-gas shift reactor prior to supplying it to the fuel cell. It is known that the so-called preferential oxidation (PrOx) reaction conducted in a suitable PrOx reactor can further reduce the CO content of H2-rich reformate exiting the water-gas shift reactor. The PrOx reactor comprises a catalyst bed operated at a temperature that promotes the preferential oxidation of the CO by the O2 in air, in the presence of the H2, without consuming substantial quantities of the H2. The PrOx reaction is: CO+xc2xd O2xe2x86x92CO2.
Often, the O2 required for the PrOx reaction will be about two times the stoichiometric amount required to react the CO in the reformate. If surplus O2 is utilized, then excessive consumption of H2 results. Alternatively, if the amount of O2 is less than the stoichiometric amount needed, insufficient CO oxidation will occur. The PrOx process is described in a paper entitled, xe2x80x9cMethanol Fuel Processing For Low Temperature Fuel Cellsxe2x80x9d published in the Program and Abstracts of the 1988 Fuel Cell Seminar, Oct. 23-26, 1988, Long Beach, Calif. and in U.S. Pat. No. 5,271,916 to Vanderbourgh and U.S. Pat. No. 5,637,415 to Mester, iner alia. U.S. Pat. Nos. 5,637,415 and 5,316,871, are each incorporated herein by reference.
Generally, PrOx reactors may be either (1) adiabatic, (i.e., where the temperature of the catalyst is allowed to rise due to oxidation of the CO), or (2) isothermal (i.e., where the temperature of the catalyst is maintained substantially constant by removing the heat generated during the oxidation of the CO). The adiabatic PrOx process typically includes a number of sequential stages that progressively reduce the CO content to avoid excessively high temperatures in one stage, damaging the catalyst. Thus, temperature control is very important in adiabatic systems. The increased temperature may also result in a reverse water-gas shift reaction (RWGS) which increases the CO content. The isothermal process can produce the same CO reduction as the adiabatic process, but in fewer stages (e.g. one or two stages) and without the concern of a reverse water-gas shift reaction.
In summary, within a PrOx reactor there are three main reactions. These include the desired CO oxidation (CO+xc2xd O2xe2x86x92CO2), H2 oxidation (H2+xc2xdO2xe2x86x92H2O) and the reverse water-gas shift (H2+CO2⇄CO+H2O). As discussed above, the CO oxidation reaction is desired because it reduces the CO content. However, CO oxidation and H2 oxidation directly compete for the available O2. Both reactions are exothermic, with the former being slightly more exothermic. The reverse water-gas shift reaction is an equilibrium reaction, generally occurring after all of the O2 has been consumed. The reverse water-gas shift reaction is dependent upon CO concentration and temperature (i.e., low CO content and high temperature are more favorable for CO formation). Because both the CO and H2 oxidation reactions are exothermic, favorable conditions are created for a reverse water-gas shift reaction. Thus, good temperature control is essential within the PrOx reactor to prevent a reverse water-gas shift reaction. One critical time point is during start-up of the PrOx reactor, during which PrOx reactors are prone to allowing poisonous CO to enter the fuel cell stack.
Therefore, it is desirable in the industry to provide an improved PrOx reactor for reducing CO concentration within a fuel reformate stream. The PrOx reactor should enable quicker light-off during start-up and limit any RWGS reaction.
Accordingly, the present invention provides a preferential oxidation reactor for reducing carbon monoxide within a reformate stream passing therethrough. In one embodiment, the preferential oxidation includes at least first and second reactor sections. The first reactor section has a first gas passage through which the gas stream flows, and a surface supporting a first catalyst for promoting oxidation of carbon monoxide in the gas stream. The second reactor section has a second gas passage in flow communication with the first gas passage and has a surface supporting a second catalyst. A control means is also provided for maintaining a first temperature within the first gas passage at a value different than a second temperature within the second gas passage. The first catalyst promotes oxidation of the carbon monoxide at a greater rate at the first temperature, as compared to a rate at which the second catalyst is operable at the first temperature to oxidize the carbon monoxide.
In another embodiment, the preferential oxidation reactor further includes a third reactor section having a third gas passage through which the gas stream flows, and having a surface supporting a third catalyst. The control means maintains a third temperature within the third gas passage at a value different than the first and second temperatures within the first and second gas passages. The reformate stream sequentially passes through the first, second, and third reactor sections with the first temperature being lower than the second temperature for enabling quicker light-off of the first reactor section and the third temperature being lower than either of the first and second temperatures for limiting a reverse water-gas shift reaction. Optionally, a catalyst density of each section may decrease through the first, second, and third sections. The selection of catalyst for the first section makes it possible to achieve oxidation of CO, or light-off, at a relatively low temperature relative to the second temperature.
In one preferred embodiment, the present invention further provides coolant flow in heat transfer relationship with the reaction sections. A first coolant flow is in heat exchange relation with the first reactor section for controlling the first temperature, a second coolant flow is in heat exchange relation with the second reactor section for controlling the second temperature, and a third coolant flow is in heat exchange relation with the third reactor section for controlling the third temperature. Preferably, the first, second, and third coolant flows respectively vary in volume.
In another preferred embodiment, the present invention further provides a first catalyst substrate operatively disposed within the first reactor section, a second catalyst substrate operatively disposed within the second reactor section, and a third catalyst substrate operatively disposed within the third reactor section. The first catalyst substrate is a lower temperature catalyst substrate than the second catalyst substrate for enabling quick light-off of the first reactor section, and the third temperature catalyst substrate is a lower temperature catalyst substrate than the second catalyst substrate for limiting the reverse gas-water shift reaction.
Finally, in accordance with a further preferred embodiment, the present invention provides a first catalyst substrate having a first reaction promoter and operatively disposed within the first reactor section, a second catalyst substrate having a second reaction promoter and operatively disposed within the second reactor section and a third catalyst substrate having a third reaction promoter and operatively disposed within the third reactor section. The first promoter enables operation of the first reactor section at a first temperature range, the second promoter enables operation of the second reactor at second temperature range and the third promoter enables operation of the third reactor at a third temperature range. Preferably, the second temperature range is higher than the first and third temperature ranges. It should be understood that the temperature ranges may overlap, provided that the operating temperature selected for each reactor section differs, in accordance with the invention. In another aspect, the temperature is selected so as to optimize the catalyst utilized for the reaction. The present invention is illustrated with reference to reaction surfaces having different catalytic characteristics. It should be understood that the invention is operable in a single reactor chamber or in a series of different reactor chambers. However, the reaction chamber, or chambers, are configured to optimize the catalyst and temperature along the flow path of the gas stream for reaction of CO therein.
One advantage of the present invention is that the preferential oxidation reactor initially provides a lower temperature oxidation reactor section for enabling quicker light-off.
Yet another advantage of the present invention is that the preferential oxidation reactor provides a final lower temperature and lower catalyst density oxidation reactor section for limiting the RWGS reaction.
Overall, the present invention provides a more efficient method and apparatus for treating a reformate stream, thereby providing a cleaner reformate as fuel to a fuel cell stack.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.